option package:shellwords

option x p tries to apply the parser p. If p fails without consuming input, it returns the value x, otherwise the value returned by p.

option x p = p <|> pure x

See also: optional.

One or none. It is useful for modelling any computation that is allowed to fail.

Examples

Using the Alternative instance of Control.Monad.Except, the following functions:

>>> import Control.Monad.Except

>>> canFail = throwError "it failed" :: Except String Int

>>> final = return 42                :: Except String Int

Can be combined by allowing the first function to fail:

>>> runExcept $ canFail *> final
Left "it failed"

>>> runExcept $ optional canFail *> final
Right 42